The marathon is where Ironman dreams die. It is very difficult to run a strong marathon after riding 112 hard miles. In fact, it is seldom done.

Consider the following example. At the 2008 Ironman Arizona, the fastest bike split was 4:26:12, and the 50th-fastest bike split was 4:55:24—29 minutes and 12 seconds, or 10.9 percent, slower. Compare this gap to the corresponding gap in the run. The fastest run split was 2:46:38, and the 50th-fastest run split was 3:20:22—33 minutes and 44 seconds, or a full 20 percent, slower.

As you can see, in the bike leg, the top 50 performers were bunched close together, whereas in the run they were spread out. This pattern is apparent in every Ironman. There are three possible explanations for this pattern:
1. The depth of running talent is less than the depth of cycling talent in Ironman events.
This explanation is implausible on its face. There is no reason whatsoever to believe that Ironman events attract stronger cyclists than runners. The athletes who compete in Ironman events are by and large the same athletes who compete in shorter triathlons, and run times are typically much more closely bunched together percentage-wise in shorter triathlons than in Ironman races.

2. Most triathletes go too hard on the bike in Ironman races and do not save enough energy for the run.
This explanation seems much more plausible than the first, but there is actually no good evidence that those athletes who produce the fastest run times in Ironman races hold back more on the bike than their fellow competitors. In fact, contrary to popular belief, elite Ironman triathletes really don’t hold back at all on the bike. If riding at 80 or 90 percent of capacity (relative to the distance of 112 miles) were normal and necessary at the elite level of Ironman racing, then you would see at least one clown fly off the front and complete the bike leg 10 or 20 percent faster than the real contenders (which would translate to 30 to 60 minutes). Even if it were suicidal, people would still do it for a moment of glory. It’s human nature. But this never happens. Why? Because elite triathletes actually ride the Ironman bike leg at something closer to 98 percent of their maximum capacity (meaning they would ride only five to 10 minutes faster in a pure 112-mile time trial).

Pacing is important, of course, but people don’t realize how great a difference there is between 98 percent and 100 percent efforts. To gain a better appreciation for the difference, go to the track and run 10K (25 laps) 2 percent slower than your 10K race pace. So, if your 10K time is 40:00, run a 40:48. I guarantee you will feel about 10 or 20 percent less miserable in the last lap at the slightly slower pace, which is why many elite Ironman racers think they are holding back 10 or 20 percent on the bike in competition when they are actually holding back 2 percent.

Riding too hard can affect subsequent run performance, but fitness trumps pacing. The less fit you are, the less your run will benefit from holding back on the bike. You could go 95, 90 or 85 percent on the bike and be shot for the marathon in any case. And the fitter you are, the less pacing matters. Craig Alexander would not run a 2:35 marathon in Hawaii instead of a 2:45 if he rode the bike leg in 4:55 instead of 4:37.

This observation leads us to the third and true explanation of the marathon meltdown phenomenon.

3. Most triathletes just aren’t well-trained enough to run a good Ironman marathon.
You start the run fatigued no matter how you pace yourself on the bike. Those who hold it together and run well simply have better Ironman-specific fitness, which enables them to run closer to their ability level despite fatigue.

With this explanation in mind, use the following tips to avoid the all-too-common scenario of running poorly in the Ironman marathon.